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TECHNICAL NOTE NUMBER 221
UNITED STATES DEPARTMENT OF AGRICULTURE UNIV cYPPITBSERVICE
FOREST PRODUCTS LAB RA CUMLENT3S DEPT-
MADISON 5. WISCONSIN REVISED Octpk.er 1952
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WEATHERING ANDI DECGYS DEPOSITORY
Two slow but important deteriorating influences against which wood
should be guarded in service are weathering and decay. Either of these
actions, if permitted, will finally cause serious disintegration of the
wood. Decay, when it occurs, is a more rapid form of deterioration
than weathering. Weathering and decay should be clearly distinguished
-from each other because theydiffer with respect to the causes producing
them, the conditions favoring them, and the methods effective in com-
Weathering is primarily due to the shrinking and swelling of wood with
continual changes in moisture content. The surface layers of a shingle,
board, or other piece of wood alternately absorb or lose moisture rap-
idly if exposed to rain and sunshine or to the ever-changing humidity of
-the atmosphere. Changes in moisture contentinside the piece, however,
lag behind those in the surface layers because of the relatively slow rate
of transfusion of moisture in wood. The lag tends to keep the interior
at a relatively uniform moisture content and a constant volume, so that
when the outside wood fibers swell and shrink they are alternately
squeezed together and pulled apart. This action results in a very slow
breaking down and wearing away of the surface fibers, and sometimes
more noticeably in "raising of the grain, checking, cracking, and split-
ting of the wood, cupping, warping, and pulling at fastenings. It may be
augmented by the action of frost, by the mechanical aorasive effect of
rain, hail, and wind, and by chemical changes in the wood substance
brought about by the action of light, moisture, and oxygen. About 1/4
inch of wood wears away from the exposed surface in a century.
Decay, on the other hand, is caused by the action of wood-destroying
fungi -- small living organisms that feed on the wood substance. The
visible effect of the attack is familiar to everyone as "rotten" wood.
Wood that's rotten, ordecaytdhas not been simply mechanically disinte-
grated as in weathering, but has been actually decomposed. Under con-
ditions favorable to the development of decay organisms, the wood may
be rendered unfit for service within a period of several months.
UNIVERSITY OF FLORIDA
3 1262 09216 7443
Weathering and decay are not usually found in r.ne same place. Wood
that is dry will not rot, because the fungi must have water to live on.
On the other hand, weathering is usually found where the boards as a
whole remain fairly dry. The surface layers of such boards periodic-
ally take up moisture, but drying occurs before the water can penetrate
to the interior of the wood.
Typical cases of weathering in wood can be found in old shingles, and in
unpainted house siding, board fences, and outdoor seating. Decay is
more common in the bottom of steps of porches, the bases of porch and
pergola columns, the lowest boards of siding that runs to the ground,
the butts of untreated posts and poles, and other wood that is used in
contact with the ground or in damp, unventilated places.
Protection against weathering can be obtained by the use of paint, var-
nish coatings, or water-repellent preparations. Such coatings, although
not impermeable to moisture, protect the wood enough to prevent rapid
changes of moisture content in the surface layers. Paint and varnish do
not preserve wood against decay. When wood must be used in places
favorable to decay, a naturally durable wood should be selected, or,
better yet, the wood should be impregnated with an effective preserva-
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